1. Field of the Invention
The invention generally relates to enhancements to data communication techniques. More particularly, the invention relates to arrangements for resolving contention and avoiding collisions, especially in a wireless data communication scenario such as those governed by IEEE 802.11x standards.
2. Related Art
Most wireless local area networks (WLANs) follow standards adopted by the Institute of Electrical and Electronic Engineers (IEEE). IEEE 802.11 refers to a group of specifications for WLANs. The initial IEEE 802.11 WLAN standard (“IEEE 802.11-1997”) was published in 1997, and was updated in 1999 (“IEEE 802.11-1999”). IEEE 802.11 defines specifications for baseband direct sequence spread spectrum (DSSS), frequency hopping spread spectrum (FHSS), and infrared communication. Sub-standards of the 802.11 standard include 802.11c (documentation of MAC procedures), 802.11d (additional regulatory domains), 302.11e (quality of service, QoS), 802.11f (inter access point protocol, IAPP), 802.11h (Dynamic Channel Selection and Transmission Power Control) and 802.11i (Authentication and Security).
In addition, two mutually incompatible standards (commonly, “wi-fi” or “wireless fidelity”) evolved: IEEE 802.11a (see “IEEE Std 802.11a-1999”), communicating with orthogonal frequency division multiplexing (OFDM) at 5 (maximum 54) megabits per second (Mbps) and 5 GHz; and IEEE 802.11b, communicating in DSSS at 11 Mbps and 2.4 GHz. Furthermore, IEEE 802.11g, being compatible with IEEE 802.11b, also operates at 2.4 GHz but at a higher speed of 54 Mbps over short distances.
In the Open Systems Interconnection (OSI) seven-layer protocol layer model, adopted by organizations working on IEEE 802 LAN standards, the lowest two layers are the physical layer (PHY layer 7) and the data link layer (layer 6). The IEEE 802 reference model breaks the data link layer (layer 6) into two sub-layers: the logical link control (LLC) sub-layer, which lies atop the medium access control (MAC) sub-layer.
Conventionally (see, for example, William Stallings, Data and Computer Communications, Fifth Edition, Prentice-Hall, Inc., 1997), the physical layer has performed such low-level functions as encoding and decoding signals, processing preambles for synchronization, and actually transmitting and receiving bits carried by a communications medium. Above the physical layer, the data link layer has traditionally performed such functions as assembling and disassembling address and error detection components of transmitted frames, providing access to layers higher than the data link layer, performing flow control and error control, and, significantly, governing access to the LAN transmission medium (see p. 365 of Stallings).
A universal challenge with shared-medium communications techniques such as WLANs, is resolving contention and avoiding collisions on the communications medium. These techniques attempt to ensure that the shared communication medium is used as fully as possible while fairly allocating use of that medium's bandwidth by competing network devices. As noted by Stallings, above, the conventional approach has been to manage access to the physical communications medium using the MAC sub-layer (IEEE reference model) in the data link layer (OSI reference model), rather than in the physical layer.
The ongoing progress of technology presents special challenges in the area of contention resolution and collision avoidance. Higher transmission speeds place special demands on networks that must support both high-throughput (HT) devices and slower, legacy network devices that use conventional CSMA/CS. Especially in “mixed” WLAN environments (those including high-throughput and legacy devices), what worked acceptably in slower legacy devices is not acceptable in higher-throughput devices. In a WLAN environment, carrier sensing has been implemented using a mechanism called CCA (Clear Channel Assessment), which involves frame length detection through the PLCP layer (described below). Collision avoidance has been accomplished through use of a NAV (Network Allocation Vector) in the MAC layer to pre-reserve the transmission medium, using a data frame itself or an RTS/CTS frame. For higher throughput arrangements, however, using the lower speed NAV reservation process costs too much overhead: at higher data rates, the overhead of sending RTS/CTS frames and of inter-frame gaps is noticeably more burdensome than in lower speed networks. Accordingly, there is a need in the art to provide a lower-overhead arrangement for achieving collision avoidance.
FIG. 1 schematically illustrates a portion of the protocol layer models that embody sending and receiving network devices that are joined by a communications medium. Beneath the “upper layers” (that are not part of the IEEE 802 specification) are found the LLC and MAC sub-layers of the data link layer described above. Below the data link layer is the physical (PHY) layer, which includes a physical layer convergence protocol (PLCP) sub-layer and a physical medium dependent (PMD) sub-layer (see FIG. 11 of “ISO/IEC 8802-11/ANSI/IEEE Std 802.11” (1999)). In “IEEE Std 802.11a-1999, (Supplement to IEEE Std 802.11-1999),” Section 17.3.6 (“Clear channel assessment (CCA)”) states that physical layer convergence protocol (PLCP) “shall provide the capability to perform CCA and report the result to the MAC. The CCA mechanism shall detect a ‘medium busy’ condition . . . ”
Conventional arrangements for resolving contention and avoiding collisions have involved a clear channel assessment (CCA) period that is set to be the transmission time of a current packet. Accordingly, such conventional arrangements do not protect “follow-on” packets in the event of a “hidden terminal.” In such scenarios, conventional arrangements must resort to transmitting additional protection frames in a backward compatible mode to attempt to protect the additional frames. Undesirably, the additional overhead of protection frames slows overall network throughput.
Accordingly, there is a need in the art for a way to fairly resolve contention and avoid collisions, especially in a “mixed” WLAN scenario in which high-throughput (HT) network devices must communicate with legacy devices over a shared communications medium.